This invention relates to an essentially solvent-free resin blend and advanced composites for substantially continuous use at a service temperature of at least 176.degree. C. (350.degree. F.) which is above that of known cured epoxy resin systems. By "essentially solvent-free" I refer to a negligible amount of solvent, typically less than 1 percent, and most preferably less than 0.01 percent by weight of the resin. Advanced composites are used chiefly in structures such as supersonic military aircraft, engine nacelles, and missile applications, because of the high specific strength and modulus of the composites. These structures are required to withstand long (several thousand hours) exposure to temperatures in the 121.degree.-232.degree. C. (250.degree.-450.degree. F.) range, and brief exposure (several minutes or less) to temperatures up to 260.degree. C. (500.degree. F.).
The composites are typically composed of graphite or boron fibers, embedded in a polymeric matrix. Less advanced composites used in industry are composed of glass fibers and may not require as high a service temperature for as long a time. The polymer matrix determines the useful limits of the composites; whether they can be made economically depends upon their improved processing characteristics. Their commercial utility depends upon their superior toughness and high resistance to damage from moisture, or stress, or impact, or radiation. Currently, the epoxies are mainly used because of their processability, but they lack adequate qualifications for high temperature performance as well as adequate resistance to moisture. Currently used resin systems which cure at relatively low temperature, such as the diglycidyl ether of bisphenol-A/meta-phenylene diamine (DGEBA/mPDA), cannot be used above 121.degree. C. (250.degree. F.).
In the specific field of "cocured" composites, reinforcing fibers impregnated with uncured resin are conformed to a preselected structural configuration and then cured. This technique may be used either to produce a complete article of arbitrary shape, or to repair a composite structure. Cocured resins are generally of low viscosity to enable them to penetrate and wet the fiber bundles. At the present time, we know of no resin which will meet the optimum criteria for a cocured resin. In particular, the inability of known resins and resin blends to maintain stiffness (in terms of shear modulus) and/or strength at temperatures much above the cure temperature, is the primary factor limiting fabrication of structural forms at low temperature, and utilizing relatively low energy, and limiting composite repairs.
There is a need for a low energy, rapid curing matrix/adhesive for fabrication of composites and for structural repair, particularly in the field of manufacture of aircraft frames reinforced with graphite fibers, and high-performance automobiles reinforced with glass fibers. Since polymerization does not proceed rapidly in the glassy state, it is not obvious how one can cure a material, particularly a resin system containing a polyimide (referred to herein as "PI" for brevity), at a relatively low temperature below about 150.degree. C. (302.degree. F.), yet produce a cured resin blend with a service temperature above 200.degree. C. (392.degree. F.). Specifically, if such a cure was proffered, it would need effectively to provide the requisite thermomechanical properties, yet be relatively insensitive to moisture.
This invention is particularly related to resin systems comprising an epoxy and an aromatic, heterocyclic, or aliphatic PI, and bisimide ("BI" for brevity) in particular. Since, in practice, the BIs are by far of greatest interest, reference hereafter will be made to BIs, it being understood that the invention applies equally to trisimides, tetraimides, and other polyimides, as long as they meet the criteria set forth in particular for the BIs. The particular BI of interest in the resin blend will depend upon the specific thermomechanical properties desired, and the elevated temperature at which the cured resin blend is expected to retain the desired properties. Aliphatic BIs generally produce cured blends for relatively low temperature service, the heterocyclic BIs and aromatic BIs producing cured blends for relatively high temperature service.
This invention is more specifically related to miscible blends of aromatic and/or heterocyclic BIs with epoxies which are known to provide relatively high temperature service (for an epoxy) in the range from about 150.degree. C. (302.degree. F.) to about 250.degree. C. (482.degree. F.) though they have an unacceptable moisture gain. The aromatic and/or heterocyclic BIs contribute high temperature stability with lower moisture gain, preferably substantially none at all.
Epoxy resins in which the monomer contains glycidyl chain ends in a polyarylene backbone have been cured with diaminodiphenylsulfone (DDS) to provide even a higher degree of cure than with tetraglycidylmethylenedianiline (TMGDA) at room temperature or slightly elevated temperature, preferably below 120.degree. F., but such cured epoxies have an unacceptably high sensitivity to moisture. They also have a lower service temperature than aromatic and/or heterocyclic BIs, and, bismaleimides (BMI), in particular.
BI resins exhibit excellent humidity resistance and have continuous use temperatures above 177.degree. C. (350.degree. F.). However they have long gel and cure times even at high curing temperatures in excess of 200.degree. C. (475.degree. F.). When used with graphite fiber, the composites have a high modulus but exhibit severe microcracking after cure, due to shrinkage caused by gelation at high temperatures. When cured with a solvent, they exhibit an unacceptable solvent retention problem which mandates a solvent-free system.
When a BI is chain extended by adding an aromatic linkage across the maleimido double bond to introduce an aliphatic secondary amine bridge, this reaction increases the molecular weight (mol wt) of the bisimide precursor and introduces a point of flexibility. One then observes easier processability, lower melting point, better solubility in solvents for BI, more controlled viscosity and reduced cured rate for better reaction control. However, where one wishes to use a wax mandrel in a lost wax molding process, the temperature of cure of the chain extended BI is still too high (that is, above the melting point of the wax). The wax runs out before the resin is cured. Such a chain extended resin is commercially available as Keramide 601 which is purportedly good for continuous service at 150.degree. C. However the presence of the Michael addition product introduces a point of thermal instability in the imide chain which suppresses thermal stability.
Another chain extended BI is M-751 in which the BI prepolymer is chain extended by increasing the chain length of the diamine by the reaction of p-phenylene diamine with m-aminobenzoic acid. In addition, the prepolymer is further extended by including an equimolar amount of an amineterminated maleimide which also reacts in situ by Michael addition. This so-called "eutectic" mixture increases mol wt, reduces the melting point, increases viscosity, and moderates the reactivity of the maleimido double bond during processing. But this chain extended BI has several points of instability (see "Bismaleimides and Related Maleimido Polymers as Matrix Resins for High Temperature Environments" by John Parker, et al. NASA, Ames Research Center in Proceedings of a conference held at NASA Lewis Research Center, Cleveland, OH. Mar. 16-18, 1983).
If the melting point of the wax is not a consideration in the preparation of a prepreg (fibers impregnated with the resin), the viscosity of known chain-extended resins at the curing temperature is so low, that the prepreg cannot hold the resin. In other words, a prepreg loses the low melting point chain-extended BI resin at the curing temperature, because the resin flows out of the prepreg before it is cured.
A solution to this problem by using a low temperature curing crosslinking agent, namely divinyl benzene, for the BI is taught in U.S. Pat. No. 4,351,932 to Street, et al. To combat the problem of too low a viscosity they add a trifunctional curing agent, also for the BI, to improve tackiness and provide a higher crosslinking density so as to make up for the disadvantages of using the divinylbenzene. To reduce microcracking they included a small amount of elastomers. Specifically, polyether sulfones and bisphenol epoxies having molecular weights from 40,000 to 120,000 have been used in an amount of 0.5 to 3% by weight, without regard for their effect in the morphology of the mixture formed. The elastomer remains dispersed within the matrix resin before the resins are cured, and to preserve the elastomeric contribution, no crosslinking agent for the epoxy ("epoxy-curing-agent") is either suggested, or used. Clearly, there is no indication that the epoxy may be cured first, and it is evident that the curing of the epoxy depends upon the temperature at which the prior art N,N'-bisimide was cured. Most of all, there is a clear emphasis on the elastomeric contribution demanded of the polyether sulfone or epoxy resin, mandating their molecular weight.
We prefer not to chain extend the BI so as to avoid the points of thermal instability produced by the Michael addition product. To get better viscosity control of the epoxy/epoxy-curing-agent as the dispersive phase, and preferably, better miscibility with the BI, and, optionally a curing agent for the BI (BI-curing-agent), we prefer to chain extend the viscous epoxy molecules with a reactive diluent ("epoxy-reactive diluent"). Of course, if the epoxy is not excessively viscous at the curing temperature of the blend, it will require no reactive diluent. Though the chain extended product of the epoxy would also be subject to analogous points of thermal instability, we reasoned that the sacrifice in thermal properties of the epoxy would not affect those of the BI, particularly if there is no direct coreaction of the epoxy groups and the BI; and, the higher the proportion of BI relative to the epoxy, the less significant would be any loss attributable to the instability of the epoxy. If the epoxy/epoxy-curing agent/BI/BI-curing-agent system still lacks adequate fluidity, a reactive diluent for the BI ("BI-reactive diluent") may be added, with the expected penalty in high temperature performance properties.
We deliberately use an epoxy which is non-elastomeric, and which must be crosslinked after it forms a fluidized dispersion, or, more preferably, a single phase of liquid epoxy/epoxy-curing-agent and an aromatic BI which is liquid at the curing temperature of the epoxy in our resin system. To form the single phase in our solvent-free resin system, it is essential that the proportions in which the relatively low mol wt epoxy, and, the aromatic BI be such that the epoxy and BI are mutually miscible at, or below, the curing temperature of the epoxy; and, preferably, the epoxy-reactive diluent, and BI-reactive diluent (if either, or both, is used), are both mutually miscible, and, each miscible in the epoxy/epoxy-curing-agent/epoxy-reactive diluent/BI blend, so as to form a single liquid phase. A single liquid phase is formed when there is no visual demarcation between the components in the liquid phase.
It is not essential that the epoxy, the epoxy-curing-agent, and the epoxy-reactive diluent, form a single liquid phase with the BI, but it is essential that the BI be homogeneously dispersed, whether the BI is a finely divided powder, or a liquid at or below the epoxy curing temperature. It will be evident that the components of the uncured resin system will be homogeneously distributed if they are mutually miscible. In those instances where the end use of the cured epoxy/BI resin can tolerate the uncertainties of dealing with the removal of moisture generated with an amine curing agent for the BI, a tetracarboxylic acid dianhydride, preferably one in which the tetravalent radical is aromatic or heterocyclic, may be used. However, in those instances where no water is to be generated, the BI, or the BI with BI-reactive diluent is thermally cured without the evolution of moisture.
We are aware of nothing in the prior art teaching that an essentially solvent-free, liquid, low mol wt epoxy, flowable under processing conditions, and curable with a curing agent at a temperature below 150.degree. C. (302.degree. F.) should be used as the dispersive (continuous) phase in which a BI is the discontinuous phase; or, that the epoxy and epoxy curing agent should be miscible with the BI, and optionally with a BI curing agent, at a temperature lower than that required to cure the BI; so that the epoxy, upon being cured can "fix" the homogeneously dispersed BI which may be cured later, simply by heating to a higher temperature than the curing temperature for the epoxy, without generating volatiles at any stage.